Dissector target



Dec. 26, 1933. P. T. FARNswoRTH 1,941,344

DIssECToR TARGET Filed July 7, 1930 PH/LO T. FRNSWOP TH. s 5 BYSQVMQ//f ATTORNEY Patented Dec. 26, 1933 UNITED STATES PATENT oFF-ICE signor to Television Laboratories, Inc., San

Francisco, Calif., a corporation of California Application July 7, 1930. Serial No. 465,814

10 Claims. (Cl. 250-27.5)

My invention relates to electron discharge apy paratus, and particularly to electron dissector tubes such as are, used for the electrical scanning and transmission of television pictures.

Objects of my invention are: to provide a dissector tube having increased sensitivity; to provide a target or auxiliary electrode for such a tube which will provide a multiplication of the received electrons by secondary emission; to provide a target whereon a secondary emitting surface may be formed after the tube has been evacuated, and in which the secondary emitting surface may be completely shielded so that heating by high frequency induction is impractical; and to provide a form of anode for receiving the secondary emission which is out of. the path of the primary radiation and which therefore will carry only the secondary electron current. Y

Other objects of my invention will be apparent or will be specically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

In the drawing, Figure 1 is an axial, sectional view of a dissector tube embodying my invention. Figure 2 isa detailed sectional View, on a greatly enlarged scale, of the target shown in the tube of Figure 1.

In general terms, the target which forms the subject matter of the present invention, comprises a shield having an aperture behind which is positioned a surface which is adapted for secondary emission of electrons. Between the aperture and the shield is an anode which substantially surrounds the pathof the electrons entering the aperture but which itself lies without this path. The anode preferably comprises a lamentary electrode which is coated originally with the material which forms the secondaryy emitting surface, andafter the tube has been constructed andevacuated the lament is heated-electrically to vaporize the secondary emitting substance and deposit it upon the target. The surface thus formed is free from contamination and highly` active as a secondary emitter.

In its preferred form, my invention comprises a tubular envelope 10 having, at one end, a planar window llthrough which the image to be transmitted may be focused upon a photo-sensitive surface deposited upon the face of a front-silvered mirror 12.` The mirror is preferably mounted by means'of clips 13 to a glass pillar 14 carried byv a stem v16 at the opposite en d ofthe ltubeffrom the window.

tive surface. The screen is supported byl a plu-l rality of wires 18 which are welded or otherwise fastened to a band 20 surrounding the stem. The wires18 also carry a cylindrical screen shield 21 whose purpose is to form a substantially equipotential space through which the electrons liberated by the cathode may pass.

The tube is preferably provided with a base 22 carrying pins 23 and 25 which connect with the anode and the cathode respectively, through leads 26 and 27, sealed through the stem 16.

Positioned in the upper or forward end of the tube, immediately behind the window 11, is the target. This comprises a tubular shield 30 which `ts Within a projecting side tube 31' and is held in position frictionally. An outer shield 32 iits over the projecting side tube and is connected to the shield 30 by a wire 33 sealed through the end of the side tube. A cap 35, over the end of the shield 30 prevents the entrance of stray electrons, and limits the discharge entering the shield to that which can pass through a small aperture 36 formed in the side of the shield facing the cathode.

Within the shield, and surrounding the' electron path between the aperture 36 and the inner rear surface of the shield, is a iilamentary anode 37, connected to leads 38, 38', which are insulated from the shield by glass beads 40. 'I'he leads are sealed out through the ends of the side tube 31, passing through an insulating button 41 mounted on the end of the outer shield 32.

The iilamentary anode 37 is originally coated with'a material which is particularly active in the emission ofsecondary electrons. Such materials are thoria, thorium, strontium oxide, and barium oxide. Any other material which is active in secondary emission, and which may be volatilized, may be used.

The tube is evacuated and the photo-sensitive surface formed` by the usual methods. After this has been done, a current is passed through the leads 38, 38 and theanode 3'7, which is thereby raised to a high temperature, Volatalizing the secondary emitting substance. This substance condenses upon the inner walls of the shield 30,

i Where it forms the active surface of the target.

In the operation of the device the shield 30 is preferably operated at the same positive potential as the anode\17 and screen 21. The lamentary anode 37 is maintained at a slightly higher positive potential. When the tube is in use, the, anode is not heated.

A wide range of potentials may be used in operating this device. Potentials as low as 150 volts between the cathode 12 and the anode 17 have been used, and potentials as high as 2,000 volts have also been tested. An intermediate potential is usually to be preferred, the limitations being the vacuum to which the tube is pumped, the insulation of the device, and the particular materials used for photo-sensitive and'secondary emitting surfaces.

Between the anode 37 and the shield 30 a potential of about 45 volts has proved to be most advantageous, since this is suflicient to attractv all of the electrons emitted from the shield without causing secondary emission from the anode itself. It is to be noted, that even though some secondary emission from the anode should occur, the emitted electrons are all drawn back to the anodewhich is at the highest potential in the system.

The particular advantages of 'the target here described reside in the fact thatthe anode is subject to no direct bombardment from` the photosensitive cathode, and that the secondary emitting surface is freshly deposited after the tube is evacuated and is therefore not subject to contamination by gasin the tube.- Experience has shown that the secondary emitting material can substantially all be evaporated from the anode. Moreover, the shield may be thoroughly heated by induction when the tube is formed and thus thoroughly de-gasied without raising the material on the anode to the vaporizing temperature. 'Ihe whole device may then be allowed tocool, and when volatilization of the secondary emitter does occur it immediately deposits upon the cold surface of the shield. f

In operation, magnetic means .are used for focusing the electrical image substantially vin the plane of the aperture 36. By electrical image;

I mean a plane in the electron stream within which the electron density at any point, bears a direct relation to the light density at a corresponding 'point in the optical image which initiates electron discharge.

I claim:

1. A target for an electron discharge device rin the plane of said image and comprising a r shield having -an aperture therein adapted to admit a portion of the electron stream forming said electrical image, a surface adapted for secondary electron emission disposed to receive the electrons admitted through said aperture, and an: anode positioned between said apertureand said surface and having an opening therein between said aperture and surface.

3. In combination, means for forming an electrical image, and a target mounted substantially in the plane of said image and comprising a `shield having an aperture therein adapted to admita portion of the electron stream forming said electrical image, a surface adapted for secondary electron-emission disposed to receive the electrons admitted through said aperture, and an anode positioned between said aperture and said surface and withoutthe path of the portion of the electron stream entering the aperture.

4. An electron discharge device comprising a shield having an aperture therein and having an inner surface adapted for secondary emission of electrons, means for directing an electron stream through said aperture onto said surface, and an anode insulated from said shield and protected thereby from said electron stream for receiving secondary electrons from said surface.

5. An electron discharge device comprising means for forming an electrical image, a tubular shield having an aperture positioned substantially in the plane of said image and having an inner surface adapted to emit secondary electrons and opposite said aperture, and an anode 1 within saidy shield and having an opening. therein alined with said aperture.

6. A target for an electron discharge device comprising a surface positioned to receive the electron discharge, and an anode adjacent said surface and comprising a vaporizable material adapted for secondary emission of electrons, said anode being adapted to'be electrically heated by the passage of current therethrough to deposit said material on said surface.l l

7. A target for an electron discharge device comprising a surface positioned to receive the electron discharge, and a lamentary anode ad.-J jacent said surface and comprising a vaporizable material adapted for secondary emission of 1 electrons.

8. A target for an electron discharge device comprising a hollow shield having'an aperture therein positioned to receive the electron discharge, and a lamentary anode comprising a vaporizable material adapted for secondary emission of electrons/and forming a loop surrounding the path of electrons entering said aperture.`

9. An electron discharge device` comprising a photo-sensitive cathode, a shield having an aper- 125 anode being adapted to'be electrically heated to 1.40

deposit said material on said surface.

PmLo T. FAnNswoR'rn. 

